Unfortunately, RIM has not exposed an API that allows apps to play the tones and alerts already stored on the phone; the app has to create it's own, and the options are limited.

The interesting thing is that the folks at RexWireless have found a way to tap into the Blackberry tunes and alerts system. ToDoMatrix adds itself as setting that can be fully adjusted in each profile. I'm not suggesting that you would want to go that far for your Timer app (which I'm about to test) but I thought that it would interest you to know.